Electronic d-band properties of gold nanoclusters grown on amorphous carbon

The electronic d-band properties are important factors for the emerging catalytic activity of Au nanoclusters of sub-5-nm size. We analyzed the d-band properties of Au nanoclusters grown on amorphous carbon supports by photoelectron spectroscopy using synchrotron-radiation light coupled with high-resolution ion scattering spectrometry which enables us to estimate the size and shape of Au nanoclusters. The d-band width (W_d), d-band center position (E_d), and apparent 5d_3/2-d_5/2 spin-orbit splitting (E_SO) were determined as a function of a number of Au atoms per cluster (n_A) and an average coordination number (n_C) in a wide range (11<n_A<1600). The W_d and E_SO values decrease steeply with decreasing n_A below ~150 owing to band narrowing which is caused by hybridization of fewer wave functions of the valence electrons. However, E_d shifts to the higher binding energy side with decreasing cluster size. The rapid movement of E_d is attributed to the dynamic final-state effect, which results in higher binding energy shifts of core and valence states due to a positive hole created after photoelectron emission. We have estimated the contribution from the final-state effect and derived the approximated initial-state spectra. Modified data, however, still show a slight movement of the d-band center away from the Fermi level (E_F) although the E_d values for Au nanoclusters are closer to E_F compared to the bulk value. This behavior is ascribed to the contraction of average Au-Au bond length with decreasing cluster size.